Ferroresonant transformer with controllable flux

ABSTRACT

A ferroresonant transformer with means for varying saturation flux capacity for controlling the transformer output. A ferroresonant transformer with at least a portion of the core structure carrying the secondary winding having two separate sections providing parallel magnetic paths for the secondary and resonant flux, with a control winding on one of the sections and a switching circuit for opening and closing the control winding. A low frequency version utilizing E and I laminations with the control winding on one of the outer legs of the E. Another low frequency version utilizing E and I laminations with the control winding encircling a portion of the secondary magnetic circuit. A high frequency version utilizing a plurality of toroid cores with the control winding on one of the cores.

United States Patent [72] Inventors Taylor C. Fletcher 2,598,617 5/1952Stimler 323/48(X) 1534 Sunnycrest, Fullerton, Calif. 92632; 3,079,546 2/1963 Kuba 323/50 Lawrence M. Silva, 4110 Mlritine, 3,1 17,274 1/1964Essinger 323/56 Portuguese Bend, Calif. 90274; Bruce L. 3,123,764 3/1964 Patton 323/56 gllllikinsongosasm Sharynne Lane, Torrance, 3,361,9561/1968 Sola 323/56(X) [21] No 814,593 Primary Examiner-J. D. Miller [22]Film Apt 8 1969 Assistant Examiner-Gerald Goldberg I [45] Patented May18, 1971 Attorney-Harris, Klech, Russell & Kern [54] FERROREmNmTRANSFORMER WITH ferroresonant transformer with means for CONTROLLABLEFLUX varying saturation flux capacity for controlling the trans- 17Claims, 16 Drawing Figs fonner output. A ferroresonant transformer withat least a portion of the core structure carrying the secondary winding[52] US. CL 323/6, having two separate sections providing parallelmagnetic paths 32 323/56, 323/60, 132 /8 for the secondary and resonantflux, with a control winding on [51] Int. Cl G051 1/38, one f th ti nsand a switching circuit for opening and 6051' closing the controlwinding. A low frequency version utilizing [50] FR ofSearch 323/6, 48, Eand laminations with the co ug] winding on on: of lhe 50, 56, 57-61,321/57, 68 outerlegs of the E. Another low frequency version utilizingE. and llaminations with the control winding encircling a portion MmCited of the secondary magnetic circuit. A high frequency version UNITEDSTATES PATENTS utilizing a plurality of toroid cores with the controlwinding on 2,519,425 8/ 1950 Barlow 323/56(X) one of the cores.

:9 in, 1111/ 1l? 1: 22 '1' PR/. c D g '1 I; /2 llln' 111] CONTROL (Z/c/Rcu/T Patented May 18, 1971 3,579,088

3 Sheets-Sheet 5 Era. 12. F1614} .307 RESONANT 305 SECONDARY 302/; 30330 1 /522 1m.- 3 W/ //Z 300 L 3/0 A 3% 3/4 3/3 304 30/? /6 7 PRIMARY 7FIG RESMW 3w $ECONDAEY CONTROL 30 I; 303 W 2 F 307 Z'SONW se'cowoAlavCONTROL 302 sec. 306

16 /N VENRJRS 72010;? C. FLETCHER, RES. RES. 307 LA wee-wee M. 5/L VA 6:55c. CON7'ROL309 BRUCE L. VV/LK/NSON 3/2 3/5 32/ 322 BY THE/2 ATTOENEYBHAZE/5, K/ECH, RUSSELL & KEEN FERRORESONANT TRANSFORMER WITHCONTROLLABLE FLUX This invention relates to ferroresonant transformersand in particular, to a new and improved ferroresonant transformerincorporating means for varying the saturation flux capacity in the corestructure for controlling the transformer output. Ferroresonanttransformers are widely used today for a variety of regulating andcontrol purposes and the basic design is shown in the US. Pat. to Sola,No. 2,143,745. A conventional ferroresonant transformer utilizing toroidcores is shown in the US. Pat. to Sola, No. 2,753,5l3.-Variousmodifications and improvements are shown in other patents to the samepatentee. It is an object of the present invention to provide a new andimproved ferroresonant transformer construction which may be utilizedwith any of the presently known ferroresonant transformers and whichprovides for saturation flux capacity variation and transformer outputcontrol.

The conventional ferroresonant transformer has a magnetic core structurewith a primary winding, a secondary winding and a resonant windingthereon. A capacitance is connected across the resonant winding. Theresonant winding may be a separate winding or may actually be thesecondary winding, with the capacitance connected across the secondarywinding. A leakage flux path is provided in the core structure. In atransformer utilizing E and l laminations for the corestructure,.a.quantity of core material is installed between theprimaryand secondary windings to provide a magnetic shunt for theleakage flux path. In transformers utilizing a'toroid for thecorestructure, at least two toroids are utilized with the primarywinding linking all of the toroids and with the secondary winding notlinking all of the toroids. The ferroresonant type of operation may alsobe achieved with a core structure having two or more separate coreunits, and several such devices utilizing a saturating transformer,without a shunt, and a series choke are shown in the U5. Pats. toSchmutz et al., No. 2,179,353, John et al., No. 2,505,620, Buie No.2,764,725, and Kohn No. 2,967,271. The theory of operation and thedetails of construction of these conventional regulating devices may beobtained from various prior art publications, including theaforementioned patents. The term ferroresonant transformer as usedherein is intended to include all such regulating devices.

In all ferroresonant transformers, the saturation flux capacity of thesecondary magnetic circuit is predominant in determining the outputvoltage for a given number of secondary turns and a given frequency.

Because of external drops such as rectifiers and wiring re sistance andbecause of other parameter changes such as frequency and saturationdrifts with temperature, means for controlling the saturation fluxcapacity and hence the output voltage is desirable. Attempts have beenmade to achieve such control in the past by using windings with DC biaswhich affect the efiective saturation flux density of the core material.These techniques require a sizeable mmf. in order to achieve control andthe cost and complexity of the required driving 1 circuits offset theadvantages of using a ferroresonant approach.

The'present invention provides for changing the saturation flux capacityof the secondary magnetic circuit in a ferroresonant transformer byisolating a portion of the secondary core material andcontrolling theflux in this section. In the present invention .the secondary magneticcross-sectional area is divided into two sections, which may be referredto as an uncontrolled section and a controlled section. Means areprovided in the controlled section to limit the maximum value of theinstantaneous flux passing through this section. Said means comprises acontrol winding encircling the controlled section and a switch elementfor shorting or opening said winding. By varying the time in the cycleat which the switch is operated, the maximum instantaneous flux in thecontrolled section can be varied from zero to a maximum value equal tothe saturation flux capacity of the controlled section. As used herein,the term saturation flux capacity means the sum of (I) the product ofsaturation flux density of the magnetic material multiplied by thecross-sectional area of the core at the uncontrolled section and (2)one-half the total change of flux in the core at the controlled section.

Accordingly, it is an object of the invention to provide a new andimproved ferroresonant transformer with a core constructionincorporating two separate sections in the secondary path, with acontrol winding on one of the sections, and switch means for opening andclosing a circuit across the control winding. A further object is toprovide transformers incorporating the invention for operation at lowfrequencies and for operation at high frequencies. An additional objectis to provide such a transformer which may incorporate the variousfeatures of conventional ferroresonant transformers and which may beconstructed utilizing present day manufacturing techniques, including Eand I laminations and toroidal cores.

Other objects, advantages, features and results will more fully appearin the course of the following description. The drawings merely showandthe description merely describes the preferred embodiments of thepresent invention which are given by way of illustration or example.Various configurations for the magnetic core structure may be utilizedincluding simple rectangular cores, E and I lamination cores, toroids,C-cores, and separate choke and transformer elements, and several specificforms are illustrated. The switch means for shorting and opening thecontrol winding typically may include a switch element and a controlcircuit for actuating the switch element. Various devices may be used asthe switch element, including a simple mechanical switch or relay, atransistor, an SCR, a Triac, athyratron, an ignitron, a gas triode, amagnetic amplifier, and a saturable reactor. Various 7 circuitryarrangements may be used as the control circuit, in-

cluding a synchronized oscillator, a phase shifter, a magneticamplifier, and a saturable reactor. Several specific examples of theswitch means are set out herein. The switching operation may beperformed every cycle or every half-cycle, and circuits for both modesare illustrated. 1

In the drawings:

FIG. I is a diagram of -a ferroresonant transformer incorporating anembodiment of the present invention;

FIG. 2 is a view of an alternative form of construction of aferroresonant transformer of the present invention suitable for use atpower frequencies;

FIG. 3 is a diagram indicating typical flux waveforms in the .controlledand uncontrolled sections and the total secondary FIG. 4 is a schematicdiagram of a circuit providing a synchronized oscillator with half-wavecontrol for use with the transformers of FIG. 2 and FIG. 11;

FIG. 5 is a view of an alternative form of construction of aferroresonant transformer incorporating another embodiment of thepresent invention;

FIG. 6 is a view of another alternative form of construction of aferroresonant transformer incorporating a preferred embodiment of thepresent invention for use at higher frequencies;

shifter control for use with the transformer of FIG. 6;

FIG. 8 is a block diagram illustrating the circuit of FIG. 7; FIG. 9 isa schematic diagram of an alternate circuit providing a synchronizedoscillator with full-wave control for use with transformers describedherein;

FIG. 10 is a diagram of a ferroresonant transformer of this FIG. 7 is aschematic diagram of a circuit providing a phase FIGS. l4, l and 16 aresectional views taken along the lines 14-14, 15-15, and 16-16,respectively, of FIG. 13.

The transformer of FIG. 1 includes a magnetic core which may beconstructed in the usual manner of interleaved or butt stackedlaminations. The various conventional transformer manufacturingprocesses may be utilized. Typical core constructions for use with E andl laminations are illustrated in FIGS. 2 and 11-16. Core constructionsfor use with toroids are illustrated in FIGS. 5 and 6 and will bedescribed later. Referring to .FIG. 1, an input or primary winding 11 isprovided on a leg portion 12 of the core 10. An output or secondarywinding 13 is provided on another leg portion 14. In the embodimentillustrated, the secondary winding also serves as the resonant winding,and a capacitor 16 is connected across the secondary winding 13. In analternative arrangement, a separate resonant winding could be providedon the leg portion 14, with the capacitor 16 connected thereacross.

A leakage flux path is provided in the core 10 between the primarywinding ll and the secondary winding 13, and comprises a magnetic shuntof core sections 20, 21 with a nonmagnetic gap 22, usually an air gap,therebetween. The elements described thus far are found in theconventional ferroresonant transformers such as described in theaforementioned Sola patents and reference may be made to the prior artfor a study of the theory of ferroresonance. A power source is connectedto the primary winding and a load is connected to the secondary winding,and the transformer functions to maintain the output voltage of thesecondary winding constant within predetermined limits for variations inload and variations in voltage of the power source.

In the core of the invention, as illustrated in FIG. 1, the secondaryleg portion 14 is divided into two sections 23, 24, which may beidentified as the uncontrolled section 23 and the controlled section 24.The uncontrolled section is designed to saturate. A control winding 25is provided on the section 24. A control circuit 26 operates a switch 27connected across the control winding 25. The control circuit serves toopen and close the switch at a particular phase angle of the outputwaveform. This phase angle is a prescribed function of a controllingsignal. If a closed loop regulating device is desired, the controllingsignal is derived from the. output voltage. While a variety of devicesmay be utilized as the switch, contemporary solid state devices arepresently preferred, such as a siliconcontrolled rectifier, and twoexamples of control circuits and switches are described herein below.

When the switch 27 is open, the control winding 25 is open circuited andthe secondary and resonant flux varies in both the uncontrolled section23 and the controlled section 24. With the switch 27 closed, the controlwinding 25 is short circuited and further variation of flux in thecontrolled section is prevented. Since the flux in the controlledsection is maintained constant by the action of switch 27, the totalsaturation flux capacity is reduced as compared to the total saturationflux capacity when the switch is closed for a minimum duration; the timerequired for the capacitor ring-over cycle.

In order for the device to operate in the ferroresonant mode theuncontrolled section must saturate. During periods when the mmf. acrossthe controlled section exceeds the mmf. required to maintain the desiredflux in this section the switch must be shorted or closed. The maximumswitch closure time is a half-cycle and this condition corresponds tothe the minimum total secondary saturation flux capacity. Sinceincreasing the switch closure time causes a reduction in total secondarysaturation flux capacity, a corresponding reduction in the outputvoltage is obtained. Therefore by opening or closing the circuit acrossthe control winding, the saturation flux capacity and hence the outputvoltage may be changed.

Prior to closing the switch and shorting the control winding the fluxvariation in the controlled section follows the instantaneous secondaryvoltage. The magnitude of the flux rate of change in the controlledsection is determined by the instantaneous secondary voltage and theratio of the reluctances of the two sections. After the switch in thecontrol winding closes, and thereby shorts the control winding, the fluxin the controlled section remains constant because any further changesin flux are prevented by the induced mmf. in the short-circuited controlwinding.

As a consequence of shorting the control winding and forcing the flux inthe controlled section to remain constant, the rate of change of flux inthe uncontrolled section must then increase to a level sufficient tomaintain the voltage that existed in the secondary winding at the timeof switch closure.

The secondary voltage prior to and after switch closure at time t (FIG.3), must be identical because the resonant capacitor across thesecondary magnetic circuit prevents any instantaneous change of voltage.The instantaneous variation of fluxes in the two sections and thecontrol action of the switch and control winding is indicated in FIG. 3.In the drawing, t, and t are the times when the fluxwaveforms passthrough zero, is the time when the switch is closed, and I is the timewhen the switch is opened.

After the switch closes, the flux in the uncontrolled section continuesto increase until the magnetic material in the uncontrolled sectionsaturates. When this occurs, at time 1 the secondary resonant windingreflects a low impedance to the resonant capacitor and initiates aring-over cycle that inverts the voltage on the secondary and resonantcircuits. As a result of the inversion of the secondary voltage by thecapacitor ringover cycle the rate of change of fluxes reverses and theflux in the uncontrolled section begins to decrease. At time 2 when theflux in the uncontrolled section decreases to a value approximatelyequal to the magnitude of flux that existed at the time of switchclosure, the switch is opened. With the switch open the flux in bothsections can now change. The total rate of change of flux in bothsections is again determined by the instantaneous value of the secondaryvoltage, and the flux division between the two sections is again givenby the ratio of the reluctances. After switch opening the magnitude ofthe rate of change of flux in the uncontrolled section decreases sincethe rate of change of total flux before and after switch opening must beidentical.

The process then repeats itself in the following half-cycle with thepolarity of fluxes and voltages being reversed.

The peak value of the total flux is equal to the peak value of the fluxin the controlled section plus the peak value of flux in theuncontrolled section.

By varying the time of switch closure the, peak flux in the controlledsection is varied, since the flux in this section increases up the timethe switch is closed. If the switch is closed at the time of flux zerocrossing the peak flux in the controlled section will be approximatelyzero. If the switch is closed at .the time the secondary flux waveformhas a maximum, the

peak flux in the controlled section will be a maximum and will be equalto the saturation flux capacity of the controlled section.

The peak value of the flux in the uncontrolled section is not affectedby varying the switch closing point since the uncontrolled section isdriven into saturation each half-cycle.

As a consequence, the peak value of the total secondary flux will varyas the peak value of the controlled flux.

Since the average secondary output voltage is directly proportional tothe peak value of the total flux, the magnitude of the average secondaryoutput voltage can be varied by varying the length of the interval whenthe switch is closed.

Since the device of this invention is operating in the ferroresonantmode and hence the secondary voltage does not ring over until theuncontrolled section saturates, the shorting or opening of the controlwinding does not introduce a discontinuity in the output voltagewaveform. The only effect of shorting the control winding is to cause achange in the amplitude of the waveform over the entire half-cycle.

For operation of the transformer as a voltage regulator, a smoothcontrol between the two conditions is desirable. This may be achieved byshorting the control winding for a certain portion of each cycle or eachhalf-cycle of the primary supply frequency. By varying the duty cyclebetween open circuit and closed circuit conditions, the output may besmoothly adjusted from minimum to maximum voltage. Suitable circuits foreffecting this control'are illustrated in FIGS. 4 and 7 and 9.

FIGS. 4 and 7 are half-wave control circuits and operate once per cycle.FIG. 9 is a full wave control that closes the switch each half-cycle. i

The ferroresonant transformer of the invention circulates sizeable mmf.in the control winding. One advantage of the transformer of the presentinvention is that this mmf. is generated by the transformer in the formof induced current and the control circuit must merely contain a switchcapable of carrying this current. No separate power supply is requiredto supply the mmf. in the control winding.

When the controllable ferroresonant transformer of the invention is usedas a closed loop regulator, a method for determining when in the cycleto close the control winding is desired. Most existing phase controlmethods are troubled with the fact'that the instantaneous phase angle ofthe ferroresonant transformer varies with load changes and varies duringdynamic voltage variations in the output. Such phase variations cancause severe instabilities in a phase controlled loop.

A more satisfactory mode of control is the type embodied in the circuitof FIG. 7. In this circuit the control winding voltage is integratedwith respect to time and the switch across the control winding is closedwhen this integrated value reaches the required level. This level isdetermined by comparing a signal proportional to the output voltage witha reference voltage and amplifying the error signal. By varying theproportionality relationship between the output voltage and thereference voltage, the output voltage amplitude can be adjusted, as byresistors 109 and 110. The integrator may be reset at the time the shortcircuit is applied to the control winding so that the integrator will beready for the next halfcycle.

In some applications, it is only necessary to short the control windingonce every cycle instead of every half-cycle because the magnetic corewill automatically saturate at the desired time during the otherhalf-cycle. This occurs when a core with square loop material is used inthe controlled section of the transformer. Under these conditions, theintegrator of the control circuit may be reset at any time between theapplication of the short circuit and the beginning of the next cycle. Avariety of integrators may be utilized, such as a resistor and capacitorcircuit, a Miller integrator, an operational amplifier connected as anintegrator, a magnetic amplifier, and the like.

Turning now to the embodiment of FIGS. 2 and 4, a core 30 is fomted ofbutt stacked E and I laminations 31, 32, respectively. A primary winding33 is positioned about the center leg 34 of the stack of E laminations.A shunt 35 is positioned between the center leg 34 and the outer leg 36and a similar shunt 37 is positioned between the center leg 34 and theouter leg 38. These shunts may be conventional in construction andtypically each comprises a stack of laminations.

A'secondary winding 41 is positioned about the center leg 34 and acontrol winding 42 is positioned about portion 44 of the outer leg 38 sothat the portion 44 functions as the controlled section of the core andthe portion 43 of the outer leg 36 functions as the uncontrolled orsaturating section of the core. In the embodiment illustrated, the leg36 is reduced in cross-sectional area at 43 for the purpose ofassuring-saturation in this section and the leg 38 is reduced incross-sectional area at 44 for the purpose of minimizing the inducedvoltage in the control winding.

In the diagram of FIG. 4, the resonant capacitor 48 is connected acrossthe secondary winding 41. The AC output volt- 7 plying holding currentto SCR 53. A trigger diode 57 is connected between the junction point 58of the integrator and the control element of the silicon-controlrectifier 53. A resistor 59 is connector from the silicon-controlrectifier control element to the point 50 to provide a load to groundfor the trigger diode. The trigger diode 57 passes substantially zerocurrent until voltage thereac'ross builds up to a given level, afterwhich the diode conducts with a relatively low impedance. A typicaldiode would be an MPT 28.

In operation, the control rectifier 53 and the diode 57 are initiallynot conducting. As the output voltage of the transformer builds up, thecapacitor 55 is charged through the resistor 54. When the voltage levelat point 58 reaches a predetermined value, the diode 57 conducts,discharging the capacitor 55 into the control rectifier 53 and turningthe con-.-

trol rectifier on. The control rectifier remains conducting until theend of the half-cycle. when it automatically turns off. The operation isrepeated for every alternate half-cycle.

The resistor 54, the capacitor 55 and the diode 57 function as arelaxation oscillator which normally runs in synchronism with the linefrequency under equilibrium conditions. If the output voltage of thetransformer increases, the DC voltage at points 49, 50 increases and thefrequency of the relaxation oscillator increases, resulting in anearlier shorting of the control winding 42 and a reduction in the outputof the transformer which in turn causes the oscillator to return tosynchronism. Similarly, a reduction in the transfonner output causes areduction in frequency of the oscillator and a later closing of theswitch 53 with a subsequent increase in transformer output voltage.

A preferred embodiment of this invention is illustrated in FIGS. 11-16,and may be used with the control circuit of FIG. 9. FIG. 11 is anoverall view of the complete transformer assembly which includes amagnetic core 300, a primary coil 301, a secondary coil assembly 302,shunts 305. and 306 and gaps 303 and 304 (FIG. 15). The secondary coilassembly includes a resonant winding 307, a secondary winding 308 andcontrol winding 309 (FIGS. 13 and 16). The secondary magnetic circuitincludes a controlled section 310, and uncontrolled sections made up of311 and 312 (FIG. 16). To assure saturation in the uncontrolled section311 and 312, a window 315 is cut in the tongue of the uncontrolledsection (FIGS. 12, 13 and 16). This window 315 reduces thecross-sectional area of the uncontrolled secondary magnetic circuitrelative to the cross-sectional area of the outer legs 314 and 313and'the total primary core cross-sectional area of core tongues 316 and316a and 310.

The control winding 309 encircles the controlled section 310. Theresonant winding 307 and the secondary winding 308 encircle boththe-controlled core section 310 and the uncontrolled core section 311and 312. The control winding 309 is encircled by secondary coil 308 andthe resonant coil 307 encircles both the secondary winding 308 and thecontrol winding 309 (FIG. 16).

FIG. 15 is a view through the magnetic shunt structure. Shunts 305 and306 appear in plan view and have an L-shape to provide magnetic fluxshunting of the primary flux existing in the primary magnetic circuit316 and 316a and the controlled section core tongue 310. The primarycoil 301 encircles both core tongues 310 and 316, 316a. If thecross-sectional area of tongue 310 is small relative to thecrossssectional area 316, 316a, then the shunts 306 and 305 may vbestraight sections that only extend the length of the primary core tongue316, 316a.

FIG. M is a view showing the primary coil structure. The primary coil301 encircles both the controlled section core tongue 310 and theprimary core tongue 316, 3160.

The core 300 of the transformer structure is assembled from modifiedstandard E andl laminations. The laminations in stack 320 are onlyencircled by the resonant winding 307 and secondary 308 winding andconsist of standard B's and Is with a window 3115 cut at the back of theE. In the stack 321, space for the control winding 309 is obtained bycutting off a portion of the tongue of a standard E lamination. Thestack 322 is made from the same standard E and l laminations with thecenter leg 310 of the E reduced in width to receive the control winding309.

A particularly simple form of full-wave control for the transformer ofFIGS. 11-16 is shown in FIG. 9. The circuit of FIG. 9 utilizes a Triacfor the switch element. Since the Triac operates each half-cycle, thenominal frequency of the relaxation oscillator must be twice thetransformer operating frequency.

Diodes 200, 201, 202, and 203 form a rectifier bridge which rectifiesthe output of the transformer. This rectified AC is applied to resistor204 and the anode of rectifier 205. The cathode of rectifier 205 isconnected to capacitor 206. This capacitor filters the rectified signalto DC. This DC in turn supplies current through resistor 208 to theZener diode 211 which results in a constant potential appearing on thecathode of the Zener 211. The DC appearing on capacitor 206 also causesresistor 207 to supply current to capacitor 209 which charges until thefiring point of the unijunction 212 is reached. At this point, theunijunction switches to a conducting state which causes the charge oncapacitor 209 to be delivered to the control electrode of the Triac 213which in turn causes the Triac to conduct, shorting the control winding.

The function of resistor 204 and Zener 210 is to supply a synchronizingpulse to the B2 electrode of unijunction 212. This is accomplished asfollows. The rectified voltage out of the bridge is not filtered becauserectifier 205 isolates this point from the filter capacitor 206. As aresult, when the AC voltage into the bridge crosses through zero, thevoltage out of the bridge drops to zero which stops the current flow inresistor 204. At this time, the voltage on the B2 electrode of theunijunction 212 is given by the Zener voltage of Zener 21! minus theZener voltage of Zener 210. During the remainder of the half-cycle, whenthe voltage is high, current flows in resistor 204 which causes theZener 204 to become forward biased. The voltage on the B2 electrode ofunijunction 212 is then given by the Zener voltage of Zener 211 plus theforward voltage of Zener 210. This latter voltage appears during most ofthe half-cycle but during the zero crossing period of the AC, thevoltage momentarily drops to the former level resulting in a negativepulse on the B2 electrode of unijunction 212.

Since the firing point of the E electrode of unijunction 2ll2 is a fixedpercentage of the voltage on the B2 electrode, reducing the B2 electrodepotential reduces the firing point of the E electrode. This will causethe unijunction to fire at this time if capacitor 209 is charged to apotential near the firing level when the voltage on the B2 electrode isat its high level.

In operation, resistor 207 is adjusted such that capacitor 209 willcharge to the firing level of the E electrode of unijunction 212 inone-half cycle of the AC period when the DC voltage on capacitor 206 isat the desired potential. If the voltage is too high, the capacitor 209charges faster, causing the firing angle of unijunction 212 and Triac213 to advance which in turn reduces the voltage on capacitor 206 untilequilibrium is established with the voltage on capacitor 206 at theproper potential for capacitor 209 to charge to the firing level inone-half cycle. Likewise, if the voltage on capacitor 206 is too low,the charging time of capacitor 209 lengthens which retards the firingangle until equilibrium is again established as before.

In the event the transformer is unable to supply the desired voltage,the firing angle is delayed until the following zero crossing of the ACwaveform is reached. At this point the previously mentionedsynchronizing pulse causes the unijunction to fire which prevents anyfurther delay in the firing angle and the resultant loss ofsynchronization. A ferroresonant transformer constructed as illustratedin FIGS. l1--16 and operated with the circuit of FIG. 9 at 60 Hz. as avoltage regulator provided substantially constant output voltage (i.e.,10.3-volts variation at 166.6-volts output) over the range of no load tofull load for input voltage variations in the range of 100 to 135 volts.

FIGS. 5 and 6 illustrate alternative constructions for the transformerof the invention particularly suitable for operation at higherfrequencies, such as 20 kHz., and utilizing a plurality of toroids forthe magnetic core structure. In FIG. 5, a primary winding 65 is linkedthrough toroids 66, 67 and 68. A secondary winding 69 is linked throughthe toroids 67 and 68. A control winding 70 is linked through the toroid68. The resonant capacitor 71 is connected across the secondary winding,and the control circuit 72 provides for closing the switch 73 across thecontrol winding. The cores 67, 68, comprise the secondary portion of thecore structure, with the core 67 corresponding to the uncontrolledsection 23 of FIG. 1 and with the core 68 corresponding 'to thecontrolled section 24 of FIG. 1. The operation of the transformer willbe the same as described in conjunction with the transformer of FIG. 1and the transformer of FIGS. 2-4.

FIG. 6 illustrates an alternative form for the transformer of FIG. 5.The toroid 67 is replaced by two toroids 67a, 67b to provide the desiredamount of magnetic material. A compensation winding 74 may be providedon one of the toroids, here the toroid 66. The compensation winding isused in the same manner as in the conventional ferroresonant transformeras described in some of the aforementioned patents. The toroids of FIGS.5 and 6 may be formed as unitary molded elements, or pairs of C-cores,or wound ribbons, or in other forms as desired.

FIG. 7 illustrates the use of a ferroresonant transformer as shown inFIGS. 5 and 6, as the output transformer in a transistor invertercircuit. A typical inverter utilizes a pair of transistors connected inseries opposition across a primary winding of an output transformer,with the AC output appearing at a secondary or load winding of thetransformer. The DC power source is connected to the junction point ofthe transistors and to a center tap of the primary winding, and a drivecircuit provides a drive current to the transistors for turning thetransistors off and on. The drive circuit may be energized from awinding on the transformer providing a self oscillating inverter or thedrive circuit may be energized from an external source. The twotransistors operate as switches with first one and then the other beingclosed to connect the DC source current through the transformer primarywinding alternating in the positive and negative directions to providethe AC voltage on the transformer. This basic inverter circuit is wellknown and several variations thereof are shown in US. Letters Pat. Nos.2,990, 5 l9; 3,256,495; 3,317,856; and 3,405,342.

The output of an inverter is connected to the primary winding 65. Thesecondary winding 69 is connected to a fullwave rectifier comprisingdiodes 81, 82 and a filter comprising a capacitor 83 and an inductance84, to provide a DC output at the terminals 85, 86. The resonantcapacitor 71 is connected across a portion of the secondary winding 69.

The DC output voltage is connected to a control circuit via lines 91,92. The control circuit 90 includes an integrated circuit component 94which provides a reference voltage and a DC amplifier, a threshold ortrigger device in the form of an unijunction transistor 95, a switch inthe form of a silicon-controlled rectifier 96, and an integratorcomprising a resistor 97 and a capacitor 98.

The function of the control circuit is to vary the area under thevoltage vs. time waveform of the control winding 70. The reset periodoccurs in the following half-cycle and will exhibit the same area asthe. controlled area because a magnetic device will not support a DCunbalance. As a. result, it is only necessary to control during one-halfcycle out of every cycle. Since the prime objective is to control thearea, a means of controlling the firing angle as a direct function ofarea is used in this control circuit. The basic operation of the circuitis shown in the block diagram of FIG. 8.

The voltage on the control winding 70 is integrated with time by theintegrator 97, 98. Since the value of the integral is proportional tothe area to be controlled, it is only necessary to close the switch 96when the integral reaches a predetermined level. This triggeringfunction is accomplished by the the switch 96 whenthe integral exceedsthe preset threshold value. In order to obtain control however, it isnecessary to vary the area and hence the threshold value. The thresholddevice has another input which controls the threshold level inproportion to the voltage applied to that input. To provide a controlloop, the voltage applied to the other input of the threshold device isderived by comparing the output voltage of the transformer at terminals85, 86 with a reference voltage and amplifying the resulting errorsignal with a DC amplifier 94.

One circuit arrangement suitable for use as the control circuit 90 isillustrated in FIG. 7. A diode 100 is connected in series with therectifier switch 96 to enhance the reverse blocking of the switch. Diode101 is connected across the capacitor 94 of theintegrator to limit theswing during the reset period of the control winding to essentially zeroso that the integrator will start at zero for the next control interval.

When the voltage on the electrode E of theunijunction ing an impedancethat is dependent on load, line, frequency or other desired parameter,in conjunction with the control winding to obtain close loop control oropen loop compensation.

FIG. illustrates the utilization of the transformer of this invention incombination with a saturable reactor to obtain DC load compensation. I

The circuit of FIG. 10 utilizes the transformer of FIG. 1 andcorresponding elements are identified by the same reference numerals. Afull-wave rectifier 225 is connected across the transistor 95 reaches acertain percentage of the voltage on the electrode B2, usually 60 to 80percent. the device switches from essentially an open circuit betweenthe electrodes E and B1 to a verylow impedance. This action causes thecharge on capacitor 98 to be delivered to the gate or control electrodeof the rectifier96, firing the rectifier into conduction. The impedancebetween electrode B2 and electrode B1 is substantially reduced duringthis period and a resistor 102 is included as a current limitingresistor.

. The integrated circuit 94 contains a voltage regulator which suppliesa fixed reference voltage and contains a DC amplifier. A typicalintegrated circuit'may be aFairchild p. A723C. Terminals 7 and 8 are thepositive input terminals, terminal 5 is the negative terminal, terminal4 is the reference voltage output and is connected to terminal 3 whichis the noninverting or reference input for the DC amplifier. Terminal 4is used as the return point for diode 101, capacitor 98, diode 103, andrectifier 96, enabling a reverse bias to be developed on the gate ofrectifier 96 and the leakage current of transistor 95 to be bled of? byresistor 104. Diode 103 serves to limit the reverse bias on therectifier 96.

Terminal 6 of the integrated circuit 94 is the DC amplifier output andprovides the varying control signal voltage for the electrode B2 of thethreshold device 95. Resistor I08 func tions to prevent the voltage onelectrode B2 from becoming so low that the charge from the capacitor 98is insufficient to fire the rectifier 96. Terminal 2 of the integratedcircuit 94 is the inverting input to the DC amplifier and resistors I09,110 form a voltage divider which divides the output voltage down to anappropriate level for the amplifier. Terminal 9 is a terminal providedfor frequency compensation of the DC amplifier to prevent high frequencyinstability. Capacitor 11] provides this compensation. Additionalfrequency compensation is provided by resistor 112, capacitor 113,resistor 114, and capacitor 115. Capacitor 116 serves to prevent noisefrom disturbing the reference voltage on terminal 4.

A ferroresonant transformer connected as illustrated in FIGS. 6-8.operated at 20 kHz. as a voltage regulator provided substantiallyconstant output voltage (i.e., 0.06-volts variation at 30.03-voltsoutput) at no load and at full load for input voltage variations in therange of 20 to 36 volts for a 28- volt rated input.

Thus, it is seen that the objects of the invention are achieved inproviding very close control of a ferroresonant transformer utilinngflux switching and without requiring separate power supplies.

The control winding can be used in other ways. By way of example, amechanical switch may be used to change the output voltage or theoperating frequency for a fixed output voltage. By connecting a suitablefrequency sensitive impedance, such as a series resonant circuit, acrossthe control winding, frequency compensation can be obtained. Theferroresonant transformer of the invention can be used with any devicehavsecondary and resonant winding 13. The rectified output from therectifier 224 is connected through one winding of a saturable reactor226 to the load 227. The other winding of the saturable reactor 226 isconnected across the winding 25.

The saturable reactor 226 offers a low impedance to the control winding25 when it is saturated and a high impedance during unsaturated periods.Essentially, the reactor functions as a switch. To obtain loadcompensation, the reactor windings are interconnected in a manner thatwill cause the conduction period of the saturable reactor to be amaximum at no load and a minimum at full load. With this configurationof elements it is possibleto obtain a variety of load compensationcharacteristics.

' We claim:

1. A ferroresonant transformer including a magnetic core structure, aprimary winding and a secondary winding,

wherein the core structure with the secondary winding includes twoseparate sections providing parallel magnetic paths for the secondaryand resonant flux;

a control winding on one of said sections; and

switch means for opening and closing a circuit across said controlwinding, with no current source connected to said control windingwhereby the only current in said control winding is self induced currentresulting from flux in said one section.

2. 2. A ferroresonant transformer as defined in claim 1' wherein thecore structure includes an opening therethrough, with said two sectionsforming opposite walls thereof.

3. A ferroresonant transformer as defined in claim 1 wherein said'corestructure includes stacked E and l laminations, with said two sectionscomprising the outer legs of the E laminations.

4. A ferroresonant transformer as defined in claim 1 wherein said corestructure includes at least two groups of stacked E and l laminations,with said two sections comprising the center legs of two differentgroups of said stacked laminations.

5. A ferroresonant transformer as defined in claim 1 wherein said corestructure is formed of a plurality of toroids, with said two sectionscomprising separate toroids.

6. A ferroresonant transformer as defined in claim 1 wherein said corestructure includes three sets of stacked E and l laminations assembledside by side, with the center set inverted with respect to the outersets, and with said two sections comprising the center legs of the outersets, respectively.

7. A ferroresonant transformer including a magnetic core structure, aprimary winding and a secondary winding,

wherein the core structure with the secondary winding includes twoseparate sections providing parallel magnetic paths for the secondaryand resonant flux;

a control winding on one of said sections; and

switch means for opening and closing a circuit across said controlwinding as a function of the secondary voltage, with no current sourceconnected to said control winding whereby the only current in saidcontrol winding is self induced current resulting from flux in said onesection.

8. A ferroresonant transformer as defined in claim 7 wherein said switchmeans includes a synchronized oscillator.

9. A ferroresonant transformer as defined in claim 7 wherein said switchmeans includes a phase shifter.

10. A ferroresonant transformer as defined in claim 7 wherein saidswitch means includes a saturable reactor.

II. A ferroresonant transformer as defined in claim 7 wherein saidswitch means provides for switching operation every half-cycle of thesupply voltage.

12. A ferroresonant transformer as defined in claim 7 wherein saidswitch means provides for switching operation every cycle of the supplyvoltage 13. A ferroresonant transformer as defined in claim 7 includingrectifier means connected across the secondary winding for producing aDC control signal voltage; and

said switch means includes a switch element and a control circuit havingsaid control signal voltage as an input for closing said switch elementwhen said control signal voltage exceeds a predetermined magnitude.

14. A ferroresonant transformer as defined in claim 13 in which saidcontrol circuit includes an integrator connected for charging from saidcontrol winding, and a trigger unit having the integrator output andsaid control signal voltage as inputs with said trigger unit actuatingsaid switch element at least once each cycle of the supply voltageconnected to the primary winding.

15. A ferroresonant transfonner as defined in claim 14 in which saidintegrator comprises a resistor and capacitor circuit.

16. A ferroresonant transfonner as defined in claim 14 in which saidintegrator comprises an operational amplifier connected as anintegrator.

17. A ferroresonant transformer including a magnetic core structure, aprimary winding and a secondary winding,

wherein the core structure with the secondary winding includes twoseparate sections providing parallel magnetic paths for the secondaryand resonant flux; and means for varying the saturation flux capacity inthe core structure by switching the secondary and resonant flux betweena first condition with flux change occurring in both of said sectionsand a second condition with substantially all of the fiux changeoccurring in one of said sections, said means for varying including acontrol winding on the other of said sections, and means for closing andopening a circuit across said control winding, with no current sourceconnected to said control winding whereby the only current in saidcontrol winding is self induced current resulting from flux in saidother section.

zgggg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,579,088 Dated May 18, 197].

Inventor(s) TAYLOR C. FLETCHER It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 10, line 11, "224" should read --225--.

Claim 2, line 1, "2.2." should read --2.--.

Signed and sealed this L th day of January 1 972.

(SEAL) Attest:

EDWARD M.FLETGHER,JR. ROBERT GOTTSCHQLLK Attesting Officer ActingCommissloner of Patents

1. A ferroresonant transformer including a magnetic core structure, aprimary winding and a secondary winding, wherein the core structure withthe secondary winding includes two separate sections providing parallelmagnetic paths for the secondary and resonant flux; a control winding onone of said sections; and switch means for opening and closing a circuitacross said control winding, with no current source connected to saidcontrol winding whereby the only current in said control winding is selfinduced current resulting from flux in said one section.
 2. 2. Aferroresonant transformer as defined in claim 1 wherein the corestructure includes an opening therethrough, with said two sectionsforming opposite walls thereof.
 3. A ferroresonant transformer asdefined in claim 1 wherein said core structure includes stacked E and Ilaminations, with said two sections comprising the outer legs of the Elaminations.
 4. A ferroresonant transformer as defined in claim 1wherein said core structure includes at least two groups of stacked Eand I laminations, with said two sections comprising the center legs oftwo different groups of said stacked laminations.
 5. A ferroresonanttransformer as defined in claim 1 wherein said core structure is formedof a plurality of toroids, with said two sections comprising separatetoroids.
 6. A ferroresonant transformer as defined in claim 1 whereinsaid core structure includes three sets of stacked E and I laminationsassembled side by side, with the center set inverted with respect to theouter sets, and with said two sections comprising the center legs of theouter sets, respectively.
 7. A ferroresonant transformer including amagnetic core structure, a primary winding and a secondary winding,wherein the core structure with the secondary winding includes twoseparate sections providing parallel magnetic paths for the secondaryand resonant flux; a control winding on one of said sections; and switchmeans for opening and closing a circuit across said control winding as afunction of the secondary voltage, with no currEnt source connected tosaid control winding whereby the only current in said control winding isself induced current resulting from flux in said one section.
 8. Aferroresonant transformer as defined in claim 7 wherein said switchmeans includes a synchronized oscillator.
 9. A ferroresonant transformeras defined in claim 7 wherein said switch means includes a phaseshifter.
 10. A ferroresonant transformer as defined in claim 7 whereinsaid switch means includes a saturable reactor.
 11. A ferroresonanttransformer as defined in claim 7 wherein said switch means provides forswitching operation every half-cycle of the supply voltage.
 12. Aferroresonant transformer as defined in claim 7 wherein said switchmeans provides for switching operation every cycle of the supplyvoltage.
 13. A ferroresonant transformer as defined in claim 7 includingrectifier means connected across the secondary winding for producing aDC control signal voltage; and said switch means includes a switchelement and a control circuit having said control signal voltage as aninput for closing said switch element when said control signal voltageexceeds a predetermined magnitude.
 14. A ferroresonant transformer asdefined in claim 13 in which said control circuit includes an integratorconnected for charging from said control winding, and a trigger unithaving the integrator output and said control signal voltage as inputswith said trigger unit actuating said switch element at least once eachcycle of the supply voltage connected to the primary winding.
 15. Aferroresonant transformer as defined in claim 14 in which saidintegrator comprises a resistor and capacitor circuit.
 16. Aferroresonant transformer as defined in claim 14 in which saidintegrator comprises an operational amplifier connected as anintegrator.
 17. A ferroresonant transformer including a magnetic corestructure, a primary winding and a secondary winding, wherein the corestructure with the secondary winding includes two separate sectionsproviding parallel magnetic paths for the secondary and resonant flux;and means for varying the saturation flux capacity in the core structureby switching the secondary and resonant flux between a first conditionwith flux change occurring in both of said sections and a secondcondition with substantially all of the flux change occurring in one ofsaid sections, said means for varying including a control winding on theother of said sections, and means for closing and opening a circuitacross said control winding, with no current source connected to saidcontrol winding whereby the only current in said control winding is selfinduced current resulting from flux in said other section.